1. Field of the Invention
The present invention relates to a correlator using a convolver to output a convolution signal of information signal and reference signal, and a communication system using it.
2. Related Background Art
A correlator uses a surface acoustic wave convolver for obtaining a convolution signal of two surface acoustic wave signals, which is recently rising in importance and very actively being investigated as a key device in spread spectrum communication.
FIG. 1 is a schematic plan view to show an example of conventional correlator of this type. In FIG. 1, reference numeral 31 designates a piezo-electric substrate of Y-cut (Z-propagating) lithium niobate or the like, 32, 33 input inter-digital transducers (IDT) which are comb electrodes formed on the surface of piezo-electric substrate 31, and 34 an output electrode formed on the surface of piezo-electric substrate 31. These electrodes are made of a conductive material such as aluminum and usually formed utilizing the photolithography technology. A signal input circuit (IN) 36 is connected to the input IDT 32, and a generator (PN) 35 for generating a pseudo noise (PN) code signal as reference signal is connected to the input IDT 33.
When an electric signal with carrier angular frequency .omega. is input from the signal input circuit 36 into the input IDT 32 in the correlator as so arranged, the piezo-electric effect of substrate causes a surface acoustic wave to propagate therein. Similarly, when an electric signal with carrier angular frequency .omega. is input from the PN signal generator 35 into the input 10 IDT 33, the piezo-electric effect of substrate causes a surface acoustic wave to propagate therein. These two surface acoustic waves propagate in mutually opposite directions on the piezo-electric substrate 31, from which a convolution signal (with carrier angular frequency 2 .omega.), which is a correlation output of the two input signals, can be obtained through the output electrode 34 by the physical nonlinear effect of piezo-electric substrate.
Letting F(t-x/v)exp{j(.omega.t-kx)}and G(t+x/v)exp{j(.omega.t+kx)} stand for the two surface acoustic waves, the nonlinear interaction produces a surface acoustic wave of their product F(t-x/v).multidot.G(t+x/v)exp(2j.omega.t) on the piezo-electric substrate 31. By providing a uniform output electrode, this signal can be integrated within a region of length of the electrode. Letting L be a length of interaction region, an output signal can be expressed as follows. ##EQU1##
Here, the integration range can be deemed as substantially between -.infin. and +.infin. if the interaction region length L is sufficiently larger than the signal length. Putting .tau.=t-x/v into above Equation (1), the following equation is obtained. ##EQU2##
In the above equation, I for integration range represents .infin.. The resultant signal is a convolution of the two surface acoustic waves. This mechanism of convolution is described in detail for example in "SHIBAYAMA, "Applications of surface acoustic wave," TELEVISION, 30, 457 (1976)."
The operation of the correlator as described is next described referring to FIGS. 2A and 2B. Let ABCDEFGH represent a pseudo noise signal 39 input from the PN signal (reference signal) generator 35 into the input IDT 33. Here, A, B, . . . each mean a code string having an arbitrary length and a code set, for example 001, 010, 011, . . . . The signal input circuit 36 also supplies a same input signal 38 of ABCDEFGH to the input IDT 32. The signal 38, 39 of ABCDEFGH has a period of T. These signals are converted by the input IDTs 32, 33 into respective surface acoustic waves, which propagate on the surface of piezo-electric substrate 31. FIGS. 2A and 2B illustratively show states of propagation. As seen from the drawings, the pseudo noise signal 39 is input into the input IDT 33 such that the order of code strings is reversed as HGFEDCBA. At the same time, the contents of each code string A, B, C, . . . are also reversed in order. The surface acoustic wave from the input signal 38 propagates from left to right while that from the pseudo noise signal 39 from right to left. Since in FIG. 2A the signals 38, 39 are not coincident with each other at the position of output electrode 34, no correlation signal appears from the output electrode 34. In FIG. 2B, the signals 38, 39 are coincident with each other at the position of output electrode 34, so that a correlation signal appears from the output electrode 34.
In this arrangement, the period T of pseudo noise signal 39 is equal to that of one bit of a signal to be transmitted. If a signal transmission speed is 64 Kbits/sec, the period of one bit is 15.6 psec. Since the speed of surface acoustic wave propagating on the surface of lithium niobate is about 3400 m/sec, a product of those is about 53 mm, which is a distance which the surface acoustic wave travels in the period of one bit, that is, in a code string unit of pseudo noise signal 39. This distance is an action length, which is equal to the length of the output electrode 34. Also, the width of output electrode 34 is set about 1.5 to 4 times greater than the wavelength of surface acoustic wave. In case the frequency of surface acoustic wave is 200 MHz, then the wavelength of surface acoustic wave is about 17 .mu.m, and therefore the width of output electrode 34 is in the range of about 25 .mu.m to 70 .mu.m.
As described above, the conventional correlators were very long in length of output electrode as compared with the width of output electrode in convolver and therefore the size of convolver was determined by the length of output electrode, which raised a problem of increase in size of correlator.